Analog-to-digital converters (AD converters) are roughly classified into three types of a successive-approximation type, a parallel comparison type, and a delta sigma (ΔΣ) type. Of these, the parallel comparison type is also called a flash type, and can obtain a digital value in a single comparison using (2n−1) comparative voltages and (2n−1) comparators for dividing a full scale of the analog signal into 2n sections. For this reason, a parallel comparison type AD converter operates at highest speed.
However, the parallel comparison type is known to have a problem that a circuit size is large. The reason is as follows. An AD converter includes three basic elements of a resistor ladder for generating a comparative voltage, a comparator group, and an encoder. The conversion accuracy of the AD converter is determined depending on the accuracy of the resistor ladder and the resolution of the comparator, and the accuracy or the resolution is improved as the circuit area increases.
That is, in the parallel comparison type, the circuit size increases in exchange for an improvement in the conversion accuracy. In other words, it is difficult to achieve high resolution with a small circuit area.
In order to achieve high resolution, it is necessary to improve voltage dividing accuracy of the resistor ladder. Generally, the accuracy of the resistor ladder depends on the accuracy of a semiconductor microfabrication technique. As a method of avoiding this, a method is used in which resistive fuses (variable resistors) are formed together with a resistor ladder, and a resistance value is adjusted by burning the resistive fuses off one by one by laser irradiation before shipment.
However, as resolution increases, a circuit size increases. Accordingly, in the above method, when the circuit operates connected with another circuit in the form of an SOC (system on chip), it is difficult to adjust a resistance value one by one. For this reason, the resolution of the AD converter is eventually limited to a range of the accuracy of a semiconductor microfabrication technique. Further, once the resistive fuse is burned off, it is impossible to adjust the resistance value after that. Thus, the voltage dividing accuracy of the resistor ladder is hardly improved.